1. Technical Field of the Invention
The present invention relates to a seal structure for flange contact surfaces in a steam cooled type gas turbine employed in a combined cycle power plant and the like which combines a gas turbine plant and a steam turbine plant.
2. Description of the Related Art
A combined cycle power plant is an electric power generating system in which a gas turbine plant and a steam turbine plant are combined, wherein the gas turbine is adapted to operate in a high temperature region of thermal energy and the steam turbine is employed a low temperature region to recover and use thermal energy efficiently. This type of power generating system has been attracting attention in recent years.
In a combined cycle power plant such as mentioned above, the method of cooling the gas turbine presents an important problem to be solved in the technical development of the combined cycle plant. As the result of trial-and-error attempts to realize a more effective cooling method there has been an evolution toward steam cooled systems in which steam obtained from the bottoming cycle is used as the coolant, and away from air-cooled systems in which compressed air is used as the coolant.
Now, referring to FIG. 6, description will be made of the behavior of the coolant steam flowing along flange contact surfaces in a conventional steam cooled system.
Four or so sets of rotor disks 1 each having moving blades 2 fixed at blade roots 3 are ordinarily disposed in the axial direction in a rotating portion of the gas turbine. An end rotor 4 is disposed in succession to the rotor disk 1 of the final stage at the downstream side, wherein the plurality of the rotor disks and the end rotor 4 are integrally connected by means of disk coupling bolts 5, to thereby constitute the rotatable portion of the gas turbine.
Further, the end rotor 4 is clamped to the rotor disk 1 disposed at the final stage by means of a flange clamping bolt 6 so that the end rotor 4 is brought into close contact with the rotor disk 1.
Coolant steam is fed to the rotatable portion of the gas turbine from a coolant steam supply system (not shown) so as to circulate through the moving blade 2 from the blade root 3 by way of a coolant flow passage (not shown) to thereby cool the moving blade 2. In such a case, the space defined between the rotor disk 1 and the end rotor 4 mentioned above is filled with a portion of the coolant steam.
On the other hand, due to insufficient inter-flange surface contact pressure caused by large areas of the flange contact surfaces between the rotor disk 1 and the end rotor 4 and surface roughness of the flange contact surfaces, minute gaps are formed between the rotor disk 1 and the end rotor 4. Hence, steam leaks develop through these minute gaps, as indicated by arrows in FIG. 6.
In a steam cooled type gas turbine such as described above, leakage of the coolant steam leads directly to degradation of the operating performance of the turbine. Consequently, a structure which suffers no leakage of the coolant steam is indispensable requirement.
On the other hand, it is noted that in gas turbines implemented by assembling a large number of parts, there are many flange contact surfaces such as the joining portions of the rotor disks, and further in those portions where a buffering sheet such as a gasket and the like can not be inserted, particularly in those portions having a large diameter where the area of contact increases, it is very difficult or impossible to ensure sufficient surface contact pressure for sealing.
As can be seen from the above, preventing leakage of the coolant steam along the flange contact surfaces presents a very important problem.